A variety of substances has been used over the years to repair damaged teeth. The best known currently include silver amalgams, which are frequently first encountered at an early age as a filling for small cavities, even in deciduous teeth. Gold alloys are a particularly valuable filling material, used frequently when a tooth has been considerably damaged, such as after several cavities have occurred and a lot of tooth must be restored. Frequently, for example, several smaller extending cavities, e.g. in the occlusal surface, will be combined and a restoration made with a single gold inlay or onlay. The gold alloys have gained an excellent reputation for strength, reliability and long life in service. However, both the gold alloys, and the other metals, such as the stainless steels, which have been technically successful in dental reconstructions and crowns, do not impart a natural tooth appearance.
Gold and other metallic-looking restorations are used for molars and teeth which are not immediately open to view when the wearer opens his mouth or smiles. For anterior teeth, however, current practice is to use materials closer in appearance to natural teeth. These are known colloquially as porcelain or plastic fillings. They are composite materials characterized by containing usually predominantly inorganic materials, normally finely divided powders, inert to the oral environment, bound together with polymeric material. The inorganic materials are frequently finely-ground fuxed oxides, particularly glasses, or crystalline quartz, while the polymer moiety is commonly a polyacrylate. These composite systems are available, for example, as pastes which are polymerized in situ, after having been activated, e.g. by adding a catalyst to initiate a polymerization reaction, just before being placed in the prepared tooth.
Fillings and restorations of this kind can be made to look much like natural teeth. In particular, the color of the restoration can be adjusted to a shade quite close to that of a patient's natural teeth by tinting with pigments. In addition, the translucency or pearlescence of the natural tooth can also be approximated through adjustment of the relative refractive indices of the materials used in the restoration. When color and refractive indices are well matched, it is possible to obtain a restoration that is barely preceptible to the glance. However, the attainment of good color and overall appearance is very difficult to achieve in practice. This is particularly true when one desires also to optimize other features of a good restoration, particularly radiopacity.
It is highly desirable for a filling or other restoration to be radiopaque, for it is by X-ray examination that a dentist determines whether or not a filling remains sound. From radiographs a dentist determines the condition of a filling, e.g. whether it has cracked, or whether decay is occurring at the interface between the tooth and the filling. Fillings and restorations which are made of metal are readily observable in X-rays. Fillings of the porcelain/plastic art are not observable by X-rays unless they have radiopaque materials therein.
Currently, dental filling materials are rendered radiopaque by incorporating barium into the inorganic powder moiety of the filling material. The most effective radiopaque agents are elements of high atomic number (i.e. the "heavy elements" of the periodic table); it is unfortunate, however, that most of these elements are either radioactive or toxic, such as thorium or lead. Barium is toxic also, but in certain medical uses it is present in a form so highly insoluble that the body is unable to metabolize enough of it to become intoxicated. In dental applications barium glasses have been used as components of dental restorations, on the hypothesis that barium ions within the structure of a glassy matrix will not be available to oral fluids (saliva, beverages, etc.) and will not, therefore, pose a problem of toxicity. Examples of the use of barium glass in dental restorations can be found in U.S. Pat. Nos. 3,801,344; 3,808,170; 3,826,778; 3,911,581; 3,959,212; 3,975,203 and 4,032,504. Unfortunately, in practice, the barium glasses are not as stable as had originally been hoped, and they have not, therefore, found favor in the art on account of the risk they pose of poisoning the patient (see, e.g. U.S. Pat. No. 3,971,754). A further problem encountered with the barium glasses is that of matching refractive indices to that of the other components of the restoration. For example, it would be desirable to use components with refractive indices in the range of about 1.5 to 1.6 (so as to closely match the refractive index of commonly used organic binders) but most barium glasses with refractive indices in this range are unsuitable for dental use according to U.S. Pat. No. 4,032,504. It is difficult, therefore, to prepare restorations containing barium glass which present an unobtrusive appearance when used for anterior surface repair. An additional problem of the barium glasses is their alkalinity. Typically, barium glasses show alkalinity values of pH 9 or greater, whereas a pH of 7 is preferred. Highly alkaline fillers appear to degrade the siloxane coating resulting from etching of the prepared tooth cavity and also cause rapid decomposition of any peroxide catalyst present in the dental restorative composition during storage.
Recent efforts in the field of dental restoration materials have resulted in the use of fillers other than barium-containing compounds as an X-ray detectable component. For example, U.S. Pat. No. 3,971,754 describes the use of certain oxides or carbonates, particularly those of lanthanum, strontium, tantalum and, less usefully, hafnium. These salts are mixed with glass-making components at the time the glass is made, yielding a lanthanum, strontium, tantalum or hafnium glass which possesses a measure of radiopacity. U.S. Pat. Nos. 3,973,972 and 4,017,454 describe glass ceramics which possess both a low coefficient of thermal expansion (an advantage in dental fillings) and a useful degree of radiopacity, by virtue of a high content of rare earth elements, particularly lanthanum. The rare earth elements absorb X-rays in the wavelength range of 0.2-0.3 A, a range commonly available from dental X-ray machines. However, the cost and problems with availability of these rare earth fillers make them generally unsuitable for commercial use.
In another approach to preparing radiopaque composites for dental use, organic halide (e.g. an alkyl iodide) has been incorporated into plastic materials (e.g. acrylate polymers), from which molded articles are made (e.g. U.S. Pat. No. 3,715,331). However, the articles molded from such compositions lack the strength of restorations made from glass or ceramic materials.
U.S. Pat. No. 4,250,277 describes a glass composition used for crosslinking polycarboxylic acid cement, wherein the glass contains zinc oxide and a large amount of boric oxide, in addition to other ingredients. This glass, however, is too water soluble to be useful in dental restorative compositions and prosthetic devices.
U.S. Pat. No. 4,215,033 describes a composite dental material containing a glass which in one embodiment is described as single phase. However, this patent does not appear to recognize that a single phase glass containing zinc oxide can be made radiopaque. Also, the single phase glass composition described in this patent is very difficult to make. Furthermore, such glass does not contain any aluminum fluoride.